Silencers



April 11, 1961 B. D. BLACKWELL ETAL 2,

SILENCERS 4 Sheets-Sheet 1 Filed June 20, 1957 n QCQ m m\ Aprii 11, 1961 BLAcKwELL ETAL 2,979,151

SILENCERS Filed June 20, 1957 4 Sheets-Sheet 2 April 1961 B. D. BLACKWELL EI'AL 2,979,151

SILENCERS 4 Sheets-Sheet 3 Filed June 20. 1957 SILENCERS 4 Sheets-Sheet 4 Filed June 20, 1957 United States Patent SILENCERS Basil Davenport Blackwell and George Clifford Chick, Bristol, England, and Raymond Peter Cane, Alton, Ontario, Canada, assignors, by mesne assignments, to Bristol Siddeley Engines Limited, Bristol, England, a British company Filed June 20, 1957, Ser. No. 666,809

Claims priority, application Great Britain June 21, 1956 6 Claims. (Cl. 181-51) .The present invention relates to silencers and concerns silencers for reducing the noise of exhaust gas streams within which term is to be understood as included for the purpose of the present specification, the hot exhaust gases or effluent issuing from the exhaust pipe of a gas turbine engine and the jet stream issuing from the jet pipe of a turbo-jet engine. The advent of gas turbine and jet propulsion engines of increasingly high thrust has resulted in a noise problem which becomes acute when such engines are being tested in test houses or even more-SO when such engines are being tested in the open air, and one object of the present invention is to provide a compact silencer for reducing the noise created by such testing which can be fitted to an engine prior to testing in a test house or which can be easily transported from one engine to another for successive testing of the engines in the open on an aerodrome or aboard an aircraft carrier.

A turbo-jet engine under test, may or may not have a jet nozzle at the outlet end of its jet pipe. Consequently a silencer for use with the engine may have to receive the hot effluent from the jet pipe of the engine either directly or from a jet nozzle attached to the outlet end of the jet pipe. I

Whichever is the case, in order to permit proper testing of the engine the silencer must not. cause the back pressure in the pipe to differ substantially from thea back pressure designed for the engine at its thrust rating. When the testing is carried out without a nozzle onthe jet pipe, the silencer must be adapted to take the place of the nozzleby creating a back pressure in the jet pipe equal to that which would have been caused by the nozzle designed for the engine. When the testingis carried out with a nozzle fitted to the jet pipe the silencer must be adapted so as not to change substantially the design/back pressure created by the nozzle. According to the invention there is provided an exhaust gas stream silencer comprising a tubular convergent member and a tubulardivergent diffuser member enclosing the convergent member with the smaller end of the divergent member terminating at or adjacent the larger end of the convergent member, substantially the Whole'of the wall of the convergent member being so perforated as to form a multiplicity of holes for the discharge of at least part of an exhaust gas stream entering the convergent member through its larger end.

Preferably the convergent member is a conical frustum; According to a feature of the present invention, when the silencer is for attachment to the jet pipe of a turbo'jet engine in'replacement of'the usual jet nozzle, the effective discharge area of said convergent member is made equal to the rated nozzle discharge area of the engine, According to an alternative feature of the present invention when the silencer is-for use with aturbojet engine having a jet nozzle, the effective discharge area of said convergent member is made not less than one and ahalf times the rated nozzlejdischarge area of the engine. "The effective discharge areaoftheconvergent member is constituted by the sum of the effective areas of all the perforations in the member through which the gaseous stream is permitted to pass, to which must be added, in the case where the member is a conical frustum, the area of the smaller end of the member.

According to a further feature of the invention, the effective discharge area of the convergent member may be adjustable to match engines of different thrust rating. This may be achieved by the use of interchangeable throttling means at or adjacent the downstream end of the convergent member, or, according to a further feature of the invention, by the provision of throttling means at or towards the downstream end of the convergent member which throttling means is adjustable to vary the effective discharge area of the convergent member.

Four embodiments of the present invention will now be described, merely by way of example, with reference to the accompanying drawings in which:

Figure 1 is an elevation partly in section of a silencer according to the invention, for use with a turbojet engine in replacement of the jet nozzle of the engine,

Figure 2 is an elevation partly in section of another silencer according to the invention also for use with a turbo-jet engine in replacement of the jet nozzle of the engine,

Figure 3 is an elevation partly in section ofyet another silencer according to the invention, for use with a tunbojet engine having a jet nozzle, and

Figure 4 is an elevation partly in section of yet another silencer according to the invention, for use with a turbo: jet engine having a jet nozzle.

Referring to Figure l, the silencer comprises a tubular convergent member 10 enclosed by a. divergent diffuser member 11 the smaller end of the member 11 terminating at or adjacent the larger end of the member 10. The member 10 is shown mounteddirectly on the outlet end of the jet pipe 12 of a turbo-jet engine. The divergent and convergent members may be mounted independently of each other on the jet pipe so as to allow for differential expansion. In the present example however these members are mounted as hereinafter described. The member 11 diverges rearwardly from the jet pipe and completely encloses the convergent member 10, the member 11 being made of steel plate and at its smaller end being provided with an outwardly directed flange '13 by which it is bolted to the jet pipe 12. r

The convergent member 10 is in the form of a conical frustum open at both ends and is also made of steel plate. The larger or inlet end of the member 10 is of the same diameter as the discharge end of the jet pipe 12 against which it abuts, and is provided with an outwardly directed flange 14. The jet pipe also has an outwardly directed flange 15 and the flanges 13 and 14 are bolted to the flange 15 with the flange 14 sandwiched between the flanges 13 and 15. I

Substantially the whole of the wall of the member 10 is so perforated as to form a multiplicity of plain holes 18 for the discharge of exhaust gases issuing from the jet pipe 12, exhaust gas discharging from each hole 18 in the form of an outwardly directed jet. Instead of plain holes however the perforations may be of the plunged type. 1 a

It will be noted that the convergent member 10 is, in effect, a jet nozzle attached to the pipe 12 and in order therefore that substantially no excess back pressure is built up in the jet pipe by reason of resistance offered to the exhaust gases by the silencer, the holes 18 are all so selected having regard to the coeflicient of discharge of each hole 18 that the total effective discharge area of all the holes 18 together with the discharge area of the smaller end 20 of the convergent member -10 is equal'to the rated nozzle discharge area of the engine.

- in Figure 3.

Throttle means (not shown) may be provided at or towards the downstream end of the member for adjusting the discharge area of the smaller end of the member 10, and such means may take the form of an interchangeable or adjustable restrictor plate as later described, an axially displaceable exhaust bullet, or any other suitable means. For the attachment of such throttle means, the member 14 is provided at the smaller end with an outwardly directed flange 19. By throttling the flow through the smaller end of the convergent member 10 to vary the effective discharge area of the convergent member, the silencer may be arranged to match engines of different thrust rating. 7

In operation, the stream of exhaust gases issuing from the jet pipe passes into the larger end of the convergent member 10, and some of the exhaust gas passes through the holes 18 in the form of a multiplicity of small outwardly directed jets, while the remainder passes through the relatively small downstream end of the convergent member 10. The single large stream of gases issuing from the jet pipe 12 is thus split up by the convergent member 10 into a multiplicity of very small outwardly directed jets and a small axially directed jet, and this results in a considerable noise reduction of the exhausting gases.

The functions of the divergent member 11 are to deflect rearwardly any of the outwardly directed jets'which impinge on it, and also to act in the well known manner of a diffuser thereby further reducing the noise of the exhausting gases. 7

By way of illustration, in the case of an engine having a rated static dry thrust of 10,000 lbs, it has been found that the noise of the efilux from a jet pipe of 27 inches in diameter may be considerably reduced by using a silencer as just described having a convergent member consisting of a frustum 43 inches long and converging to a diameter of 8 inches at its smaller end, and a surrounding divergent rnember 63 inches long and diverging to a diameter of 40 inches at its larger end, the diameter of the holes or perforations in the convergent member being 0.5 inch.

Referring now to Figure 2, in this case the convergent member 22 has an outwardly directed flange 23 at its larger end bolted to the flange on the jet pipe, and the divergent member 24 is made with its smaller end large enough to pass over the flanges 15 and 23 and is mounted on a wheeled carriage or trolley, for example as shown In this manner the member 24 is supported independently of the jet pipe and the member 22 so as to allow for differential expansion.

The smaller end of the member 22 is flanged outwardly and strengthened as at 27 and supports a restric tor plate 28 and a hollow detachable cylindrical perforated extension piece 29 comprising an end closure member in the form of a conical cap 30 which is bolted in position as at 31. The member 22 and the extension piece 29 including its cap 30 are all perforated as at 33, the perforations in this case being of the plunged type, and the member 24 extends just beyond the apex of the cap 30 so that the member encloses all the perforated parts.

The silencer just described with reference to Figure 2 operates in a substantially similar manner to that described with reference to Figure 1, but the embodiment of Figure 2 possesses a greater potentiality for matching difierent engines of varying thrust rating by reason of the provision of a larger diameter at the smaller end of the member 22, this larger diameter being variable over a wider range by the use of interchangeable restrictor plates 28 having diiferent diameter openings. Alternatively a restrictor plate having a variable diameter opening may be used. The detachable extension piece 29 with its-end cap 30 is bolted to the smaller end of the member 22 downstream of the restrictor plate 28, and the effective discharge area of the perforations or holes 33 in the extension piece, including its end cap, is made equal to the discharge area of the smaller end of the member 22 when unrestricted by a restrictor plate. The member 22, as previously described, is designed to have a total eflective discharge area equal to the rated nozzle discharge area for the engine, the elfect of a restrictor plate, such as 28, being taken into account. The provision of the extension piece and the end cap reduces the noise of that portion of the eflluent permitted to pass through the smaller end of the member 22.

In the case where the total discharge area of the holes 33 in the member 22 is sufficient to match an engine under test and the restrictor plate such as 28 entirely blanks off the smaller end of the member 22, the extension. piece 29 and the end cap 30 may of course be dis pensecl with.

Referring now to Figure 3, there is illustrated in this figure a mobile silencer comprising a perforated convergent member 40 enclosed by a divergent diffuser member 41. The member 41 is mounted on a wheeled carriage or trolley, generally indicated at 43', so that it can be quickly moved from one engine to the next of, for example, one or more aircraft standing in the open on an airfield. The silencer is designed for use with an engine having a jet nozzle attached to the end of its jet pipe and is shown in operative position against the jet nozzle 46 of an engine to be tested the nozzle being received with clearance within the larger end of the member 40. The member 40 is supported from the member 41 at its smaller, diameter end, the member 40 having an outwardly directed flange 42 which is sandwiched between an outwardly directed flange 43 on the member 41 and an apertured baflle plate 44, the flange 43 being bolted to the baflle plate by bolts which pass through the flange 42. The baffle plate 44 has an aperture 45 which receives the end of the jet nozzle, so that a sheath of ambient cooling air may be entrained through the annular gap between the jet nozzle and the battle plate during operation. With this entrainment, the total effective discharge area of the convergent member 40 must not be less than, and is preferably equal to, one and a half times the sum of the rated discharge area of the jet nozzle and the discharge area of the annular gap between the jet nozzle and the bafiie plate.

Suitable means may be provided for braking or anchoring the trolley in position against the jet nozzle.

As has already been described with reference to Figures 1 and 2, the silencer shown in Figure 3 may be adjusted to suit engines of varying rated thrust by means of a throttlingcontrol such as a restrictor plate attached at the smaller end of the member 40.

To suit a larger diameter jet pipe without a jet nozzle, one or more perforated frusto-conical sections may be added to the inlet end of the convergent member 10 or 22 of the silencer shown in Figure l or Figure 2, provided that the total effective discharge areav of the lengthed convergent member 10 or 22 is equal to the rated nozzle discharge area forthe engine under test. Where the convergent member is lengthened the divergent diffuser member'must, of course, be interchanged. This method of adaptingthe silencer of Figure 1 or Figure 2 is, however,.limited by the maximum area of the outlet diameter of the smaller end of the convergent member 10 or 22 or, in the case of Figure 2, of the extension pieces being used. Should a still larger discharge area be required, the convergent member 22 can be made in perforated frusto-conical sections and shortened from the smaller end by removing one or more sections. With reference to Figure 2, 'this removal would permit. the replacement of the existing cylindrical extension piece by an extension piece of larger diameter and discharge area, and by restrictor plates permitting a greater flow. The exten-. sion piece itself may also be. made in sections to vary its length and therefore the discharge 'area downstream of therestrictor plate in order to match the variable area permitted by a change of restrictor plate.

The sections of the convergent member, the diverging ceive the largest diameter pipes, and may then be adapted In the case of the silencer shown in Figure 3, different diameter jet nozzles may be accommodated by interchanging the baffle plate 44, the larger diameter end of the member 40 being made large enough to receive, with the necessary clearance, the largest diameter jet nozzle likely to bemet. frusto-conical sections may be added to the inlet end of the convergent member 40 as well as interchanging the baffie plate, or a baffle plate of variable aperture may be used, the total effective discharge area of the extended member 40 being maintained at not less than one and a half times the sum of the rated discharge area of the jet nozzle and the discharge area of the annular gap between the jet nozzle and the baffle plate.

Figure 4 illustrates a mobile silencer corresponding to Figure 3, for use with an engine having a jet nozzle attached to the end of its jet pipe, and, as before is shown in operative position against the jet nozzle 46 of the engine to be tested, the nozzle being received with clearance Within the larger end of the perforated convergent member 40. In this case, however, the baffle plate 44 is omitted, and instead a bell mouthed extension piece 50 is provided bolted to the larger end of the member 40 to define with the jet pipe 47 of the engine a converging annular passage leading to the annular gap between the jet nozzle 46 and the member 40.

The bell-mouthed extension assists the gas stream issuing from the jet nozzle in inducing the entrainment of ambient air into the member 40. Ambient air so entrained joins the exhaust stream and cools it thereby assisting to reduce the noise.

The silencer of Figure 4 may be provided with a bafiie plate corresponding to the baflie plate 44 previously described with reference to Figure 3, in order to adapt it for use with an engine having a smaller diameter jet nozzle. In this case the baffie plate carries the bellmouthed extension piece, the extension piece having its smaller diameter end equal in diameter to the Widest available aperture in the baffle plate.

With the construction shown in Figure 4, or as just described with reference to Figure 4, the total effective discharge area of the member 40 is not less than, and is preferably made equal to, one and a half times the rated nozzle discharge area of the engine plus the discharge area of the annular gap between the nozzle and the convergent member 40.

If desired, water injection may be used to assist any of the silencers described, for example, by spraying water through nozzles on to the perforated convergent member.

The perforations in the convergent member and any Alternatively, one or more perforated rating of the engine under test and product a convergent member of stiffer construction for sheet of the same gauge.

The silencer may be used alone or in conjunction with other silencing equipment. Where it is used in a test house in conjunction with other silencing equipment, it

may be inserted between the discharge end of the jet pipe and for example, an air-entraining diffuser of the other silencing equipment, so that it receives the hot gases direct from the jet pipe and after causing these to traverse the convergent member discharges them through the divergent member into the air entraining diffuser of the other silencing equipment.

We claim:

l: A mobile exhaust gas silencer comprising a wheeled I carriage, a divergent imperforate diffuser member carried by said carriage andformed at one end with an inlet opening and at its other end with an outlet opening which is open to'atmosphere; and a perforated tubular member which is mounted within and connected to the diffuser member, the perforated member being formed at one'end with aninlet opening which is disposed adjacent the inlet opening of the diffuser member and having a cross-sectional area which decreases from the inlet opening in the downstream direction to form between the diffuser member and the perforated member a passage of increasing cross-sectional area in the downstream direction.

2. A mobile exhaust gas silencer comprising a wheeled carriage, a frusto-conical divergent imperforate diffuser member carried by said carriage and formed at one end with an inlet opening and at its other end with an 'outlet opening whichis open to atmosphere; and a perforated tubular member which is mounted within and connected to the diffuser member, said perforated member being formed at one end with an inlet opening which is disposed adjacent the inlet opening of the diffuser member and at its other end with an outlet opening which is disposed upstream of the outlet opening of the diffuser, and said perforated member having a cross-sectional area which decreases from the inlet opening in a downstream direc' tion to form, between the diffuser member and the perforated member, a passage of increasing cross-sectional area in the downstream direction.

3. A mobile exhaust gas silencer comprising a wheeled carriage, a frusto-conical divergent imperforate diffuser member carried by said carriage and formed at one end with an inlet opening and at its other end with an outlet opening which is open to atmosphere; and a perforated tubular member which is mounted within and connected to the diffuser member, said perforated member being formed at one end with an inlet opening which is disposed adjacent the inlet opening of the diffuser member and at its other end with an outlet opening which is disposed upstream of the outlet opening of the diffuser, and said perforated member having a cross-sectional area which decreases from the inlet opening in a downstream direction to form, between the diffuser member and the perforated member, a passage of increasing crosssectional area in the downstream direction; and a restrictor plate detachably mounted on said other end of the perforated member for restricting fiow through the said outlet opening thereof.

4. A mobile exhaust gas silencer comprising a wheeled carriage, a frusto-conical divergent imperforate diffuser member carried by said carriage and formed at one end with an inlet opening and at its other end with an outlet opening which is open to atmosphere; and a perforated tubular member which is mounted within and connected to the diffuser member, said perforated member being formed at one end with an inlet opening which is disposed adjacent the inlet opening of the diffuser member and at its other end with an outlet opening which is disposed upstream of the outlet openingof the diffuser, and

said perforated member having a cross-sectional area which decreases from the inlet opening in a downstream direction to form, between the diffuser member and the perforated member, a passage of increasing cross-sectional area in the downstream direction; a restrictor plate detachably mounted on said other end of the perforated member for restricting flow through the said outlet opening thereof, and a perforated tubular extension piece secured to the said other end of the convergent member and forming a continuation thereof downstream of the restrictor plate.

5. A mobile exhaust gas silencer comprising a wheeled carriage, a frusto-conical divergent imperforate diffuser member carried by said carriage and formed at one end with an inlet opening and at its other end with an outlet opening which is open to atmosphere; and a perforated tubular member which is mounted within and connected to the diffuser member, said perforated member being formed at one end with an inlet opening which is disposed adjacent the inlet opening of the diffuser member and at its other end with an outlet opening which is disposed upstream of the outlet opening of the diffuser, and said perforated member having a cross-sectional area which decreases from the inlet opening in a downstream direction to form, between the diffuser member and the perforated member, a passage of increasing crosssectional area in the downstream direction; a baffle plate having a circular aperture therein, which baflle plate is detachably connected to the said one end of the perforated member, the circular aperture in the baffle plate being arranged coaxially with the inlet opening in the perforated member; and a restrictor plate detachably mounted on said other end of the perforated member for restricting flow through the said outlet opening thereof. 7

6. A mobile exhaust gas silencer comprising a wheeled carriage, a frusto-conical divergent imperforate diffuser member carried by said carriage and formed at one end 8 with an inlet opening and at its other end with an outlet opening which is open to atmosphere; and a perforated tubular member which is mounted within and connected to the diffuser member, said perforated member being formed at one end with an inlet opening which is substantially coincident with the inlet opening of the diffuser member and at its other end with an outlet opening which is disposed upstream of the outlet opening of the diffuser, and said perforated member having a cross-sectional area which decreases from the inlet opening in a downstream direction to form, between the diffuser member and the perforated member, a passage of increasing cross-sectional area in the downstream direction, and a bell-mouth extension piece detachably connected to the perforated member to register with the inlet openings of the perforated member and the diffuser member.

References Cited in the file of this patent UNITED STATES PATENTS 984,212 Gray Feb. 14, 1911 1,067,200 Schlosberg July 8, 1913 1,745,492 Kelch et a1. Feb. 4, 1930 1,934,596 Fogas Nov. 7, 1933 2,248,456 Harris July 8, 1941 2,685,936 Brenneman et a1 Aug. 10, 1954 FOREIGN PATENTS 546,538 Germany Mar. 15, 1932 382,418 Great Britain Oct. 27, 1932 851,012 France Sept. 25, 1939 OTHER REFERENCES Publication, Noise Control, July 1955, pages 37-41 and Publication, Noise Control, January 1956, pages 10-14 and 65. 

